In the methods of wastewater treatment and wastewater treatment equipment, there have been some methods and devices as general pretreatment methods and pretreatment devices. For example, there are pretreatment devices which perform settling, filtration, pH control, ozone oxidation, adsorption and the like as pretreatment devices in biological treatment equipment. These pretreatment devices aim at reducing biological, chemical and physical loads on the wastewater treatment equipment in the subsequent step, so that it can be expected to reduce scale of the wastewater treatment equipment and to enhance treatment quality of treated water from the wastewater treatment equipment.
As a prior art, a treatment method and a device using nanobubbles are disclosed in Patent Document 1 (JP 2004-121962 A). This prior art utilizes such characteristics of nanobubbles as decrease in buoyancy, increase in air bubble surface area, increase in surface activity, generation of local high pressure fields, a surface active property and an antiseptic property attained by achievement of electrostatic polarization. Specifically, it has been disclosed that by associating these characteristics with each other, a contamination component adsorption function, a substance surface high-speed cleaning and an antiseptic function allow advanced cleaning of various substances with low environmental load so that the contaminated water can be purified.
As yet another prior art, a nanobubble generation method is disclosed in Patent Document 2 (JP 2003-334548 A). This prior art is composed of (1) a step of gasifying part of liquid by decomposition in liquids, (2) a step of applying ultrasonic waves in liquids, or (3) a step of gasifying part of liquid by decomposition and a step of applying ultrasonic waves.
While the properties and functions of micronanobubbles are under research and development, methods and equipment for effective generation of micronanobubbles are being researched. Generally, microbubbles are defined as microscopic bubbles with a diameter of 50 micrometers or smaller, whereas nanobubbles are defined as microscopic bubbles with a diameter of less than 1 micrometer.
However, there have been technical problems in concurrent generation of both the microbubbles and the nanobubbles. When the microbubbles are discharged into water, the water becomes white like milk. The water stays in the whitish state for a few minutes even after the generation of the microbubbles is stopped. However, if the conditions for the microbubbles generation are inappropriate, there occurs a phenomenon that the microbubbles fail to be generated, let alone nanobubbles.
In wastewater treatment, it is important to stably generate micronanobubbles in water for effective fulfillment of the functions of the micronanobubbles.